mg-repro-flight: Provide --rebuild to make variant build jobs too

mg-repro-flight: Provide --rebuild to make variant build jobs too

This allows a single command to repro a particular job with a variety
of different source code.

The implementation technique is:
  - run the build job in a separate flight, so that it can run
    with a separate task which gives its host up after the build
  - do much of the heavy lifting of runvar fiddling etc. in
    a new helper routine in cs-adjust-flight

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
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v2: Add a missing `continue' (without which everything goes quite wrong)